Beam-collimated radiation sources are desirable for many applications such as pointing (e.g., laser pointers), free-space optical communication or remote sensing, where light needs to be concentrated in a small angle in the far field. Collimated light sources are also important for applications, such as interconnections in optical communication systems, where a laser output is coupled into optical fibers and waveguides. Collimation of light sources is conventionally conducted externally using bulky and usually expensive optical components, such as lenses, or parabolic mirrors. Herein, collimation is defined as low divergence (e.g., a few degrees or less), which for semiconductor lasers includes a divergence angle substantially less than the value of the original devices without collimation (e.g., ten to a few tens of degrees). Because collimated sources provide output beams with low divergence, such sources generally do not require additional collimation lenses and meticulous optical alignment to achieve a desired beam profile and/or directionality. In situations where super-collimated beams (e.g., divergence angle much smaller than 1 degree) are required, low numerical-aperture (NA) lenses may still be used for the collimated sources, which is a cost-effective solution compared with directly using high NA lenses.
For many conventional light sources, the spatial profile of their radiation has an intrinsically large divergence angle. For example, radiation from the p-n junction of a light-emitting diode (LED) is almost isotropic inside the device. If one considers the effect of waveguiding and device-encapsulation on the light output, the divergence angle of LEDs is still very large (e.g., at least tens of degrees). For edge-emitting semiconductor lasers, the divergence angle in the material growth direction is typically large (e.g., tens of degrees). This is because the size of the laser waveguide in the material growth direction, w, is usually comparable to or even smaller than the laser wavelength in the free space, λo. When laser radiation propagates from such a confined waveguide to the free space, it diffracts into an angle that can be roughly estimated by λo/w, yielding a divergence angle on the order of 1 radian or about 60 degrees. Among semiconductor lasers, vertical-cavity surface-emitting lasers (VCSELs) are considered to be superior in beam collimation because they usually have much larger emission area compared to edge-emitting lasers. Commercial VCSELs have a divergence angle ranging from 5 to 30 degrees, but typically about 15 degrees. However, despite the smaller divergence angle achieved by VCSELs, they have an intrinsic problem of unstable output polarization.
Previously, Lezec, et al., proposed and demonstrated a passive aperture-groove structure that was capable of collimating incident visible light [H. J. Lezec, et al., “Beaming light from a sub-wavelength aperture”, Science 297, 820 (2002)]. The aperture-groove structure was defined on a suspended metal film and included a central aperture that was surrounded by periodic grooves. Lezec's results showed that a properly designed passive aperture-groove structure could have high power throughput and that the beam emerging from the structure could have a small divergence angle. These results, however, may be viewed as counter-intuitive, as wave optics suggest that light emerging from a subwavelength aperture should be essentially isotropic in the half space and that the transmission efficiency of a single subwavelength aperture should be proportional to (r/λo)4<<1, where r is the size of the aperture [H. A. Bethe, “Theory of diffraction by small holes” Phys. Rev. 66, 163 (1944)].
The beam-collimation phenomenon of Lezec's work can be understood as follows. The light coming out of the aperture couples into surface plasmons propagating along the grating. Surface plasmons are surface electromagnetic waves that are confined to and propagate along the interface between a metal and a dielectric. These surface plasmons are scattered into free space by the periodic grating grooves. The direct emission from the aperture and the reemissions originated from the scattering of the surface plasmons constructively interfere with each other, giving rise to a collimated beam in the far field.